Examples of high density, large capacity recording media include Blu-ray discs (BD), digital versatile discs (DVD), video discs, and various kinds of discs used for document storage. Such optical data recording media (“optical discs” below) are recorded with a pit and land pattern using optical memory technologies. Optical memory technology has also been adapted for storing data files.
Methods of further increasing optical disc recording density are also being studied. One such method involves increasing the numerical aperture (NA) of the objective lens used for reading and/or writing. The objective lens focuses a laser beam on the optical disc to form a light spot at the diffraction limit. Generally speaking, the energy density of the focused light increases as the beam spot diameter of the optical disc decreases. However, data recorded on write-once optical discs and rewritable optical discs is read by focusing a laser beam with less power than is required to erase the marks and pits written on the disc. The laser emission power used to read such discs is therefore limited.
To increase the data transfer rate during recording and reading, the rotation speed of the disc and channel bit rate have also been increased. In general, rewritable optical discs that conform to the DVD or BD standards have a phase change recording layer that changes between crystalline and amorphous states. Such media are recorded by focusing a powerful laser beam by means of the objective lens onto the recording film of the optical disc to raise the temperature of the recording film above the melting point and then rapidly cooling the melted spot to form a non-crystalline (amorphous) recording mark. When a laser beam that is powerful enough to raise the temperature of the recording film to near the melting point is focused on the recording film, the temperature of the recording film where the spot is focused rises above the crystallization temperature and then gradually cools in the crystallized state. By using this phase change property of the recording film and modulating the power of the laser beam using a binary recording signal (NRZI), data (recording marks) can be recorded and erased and a rewritable recording medium is achieved.
Differences in the optical characteristics, such as reflectivity, of the crystalline and amorphous phases of the recording film are used to read information from the optical disc. More specifically, the laser beam is focused at a low power level (the average read power Pave of the laser beam) on the recording film and the change in reflected light is detected to produce an analog read signal from the recorded data. A digital signal processing circuit such as a PRML (partial response maximum likelihood) circuit then digitizes the analog read signal, and an error correction circuit applies error correction and demodulation processing to acquire the desired information.
A write-once optical disc can be produced by forming the recording film using a Te—O-M material (where M is a metallic element, a dielectric element, or a semiconductor element). This type of write-once optical disc is taught in Japanese Unexamined Patent Appl. Pub. JP-A-2004-362748, for example. The Te—O-M material used as the recording material is a material containing Te, O, and M, and immediately after film formation is an alloy having a uniform distribution of Te, Te-M, and M particles in the TeO2 matrix. When a laser beam is emitted to a film made from such a material, the film melts and Te or Te-M crystals with a large crystal diameter are deposited. Differences in the optical states of the parts of the recording film exposed to the laser beam and the unexposed parts of the film can also be detected as a signal from such discs, and this characteristic can be used to render a write-once optical disc that can only be written one time.
In order to read rewritable and write-once optical discs such as described above, a high frequency modulation circuit modulates a high frequency signal of several hundred megahertz on the drive current of the semiconductor laser. This is to prevent a drop in the S/N ratio of the read signal as a result of light reflected back from the optical disc increasing noise in the laser beam.
Methods of preventing the drop of the S/N ratio of the read signal by using high frequency modulation to suppress an increase in noise caused by reflection of the laser beam are further described below.
Japanese Unexamined Patent Appl. Pub. JP-A-2004-355723 teaches a method of changing the amplitude of the high frequency signal modulated on the laser beam when reading according to the type of optical disc. The optical disc drive taught in JP-A-2004-355723 changes the amplitude of the high frequency signal modulated on the drive signal for driving the semiconductor laser according to the identified type of the optical information recording medium.
Japanese Unexamined Patent Appl. Pub. JP-A-2000-149302 teaches a method of changing the modulated frequency and amplitude of the output power of the semiconductor laser according to the operating mode of the optical disc drive, that is, whether the optical disc drive is reading or writing the disc.
If a laser beam with the small spot size needed to read or write a high density disc is used to read or write a low density optical disc, the servo signal is distorted because the spot size of the focused laser beam is small compared with the size of the recording marks and guide track pitch. To solve this problem of recording and reading at least two different types of optical discs with different recording densities, Japanese Unexamined Patent Appl. Pub. JP-A-H10-228645 teaches a method by controlling the high frequency modulated current to modulate the drive current more when reading and writing optical discs with a low recording density than when reading and writing optical discs with a high recording density.
Japanese Unexamined Patent Appl. Pub. JP-A-2003-308624 teaches a method of calculating the differentiation efficiency of semiconductor laser drive from the current driving the semiconductor laser, and setting the amplitude of the high frequency current according to the calculated differentiation efficiency. When the differentiation efficiency of the semiconductor laser varies or the differentiation efficiency of the semiconductor laser changes over time, the method taught in JP-A-2003-308624 enables always superimposing the optimal minimum required high frequency current and reducing power consumption and extraneous radiation. JP-A-2003-308624 also teaches a method of controlling the high frequency modulation means to determine the amplitude of the high frequency current appropriate to the calculated drive differentiation efficiency by selecting from among a plurality of preset high frequency current amplitude levels, and modulate a high frequency current of the selected amplitude.
When the linear velocity of the optical disc is increased in order to improve the data transfer rate of the optical disc, the bandwidth of the read signal increases and the S/N ratio of the signal decreases. If high frequency noise emitted from the circuits is a concern, a drop in the S/N ratio where the bandwidth increases can be compensated for by increasing the laser emission power when reading according to the linear velocity. However, the recorded marks or pits may be erased on a write-once optical disc or rewritable optical disc if the laser output power is increased when reading, and the reliability of the recorded data cannot be maintained.
To solve this problem, the method of the invention enables improving the S/N ratio of the read signal to reproduce information from an optical data recording medium such as an optical disc without erasing the recording marks by the laser beam when the linear velocity of the disc is increased.